Online teaching has long brought benefits and practical advantages for many levels. Along with the advantages of the flipped classroom (FC), the combination of online teaching in the FC has increased efficiency and improved student-learning outcomes. However, there are still too few research works to improve the effectiveness of IT teaching under the FC model, especially in high schools. To improve teaching efficiency as well as improve online learning performance for students, by experimental method with an experimental group and a control group, the study compares pretest and posttest regarding the learning results of 80 students in grade 10, semester 1 of the school year 2021-2022 with two stages (teaching and assessment). As for p-value = 0.000 < 0.005 of the experimental and control groups, the results of the experimental group were more effective than those of the control group. In addition, with the value of t = 6.736, the effect size of the study is quite large. Therefore, the difference between the two groups is statistically significant. The results of this study also confirm the satisfaction level of students with teachers using FC in online teaching to help students improve their learning performance as well as learning outcomes.